The present invention relates to echo cancellation in telecommunications systems. In particular, this invention addresses the unnecessary and often detrimental subtraction of synthetic echo by echo cancellers.
An echo cancellation device is a necessary component in modem telecommunications systems. In situations of imperfect impedance matching, a signal broadcast over a telecommunications network may reflect back to the generator of the signal. Since the reflected signal must travel from the location of impedance mismatching and back to the generator, the generator of the signal receives the reflected signal with a delay. This return of a reflected signal with a delay is commonly known as echo. Without echo cancellation, a telephone user would not be able to effectively communicate because the reflected signal would return the speaker""s voice to him or her after each uttered sound.
An echo canceller typically operates by generating a synthetic echo created to duplicate the echo signal received by the telephone user. The echo canceller then subtracts the synthetic echo from the incoming signal. The subtraction of the synthetic echo during normal conditions improves the performance of the telephone by diminishing the echo heard by the telephone user. Under certain conditions, however, the echo canceller can degrade the performance of the telephone. The echo canceller may falsely adapt during ringing by creating a synthetic echo when there is no significant echo signal. In addition, the echo canceller may react too slowly in generating a synthetic echo and enhance echo where there is a change from a zero echo path connection to a finite echo path connection and vice versa. For example, the echo canceller may falsely adapt when the telephone receiving the call goes off-hook.
Without ringing and off-hook detectors, the echo canceller can degrade performance of the cellular telephone when a mobile caller places a call to a landline or analog telephone. This degradation is manifested in either initial echo enhancement or severely increased convergence time which, in turn, leads to an echo that remains uncancelled.
Most echo cancellers typically utilize a double-talk detection algorithm. This algorithm is based on the assumption that the caller is silent during ringing. If, however, the mobile caller is not silent or if there is background noise, the double-talk detection algorithm may allow the echo canceller to generate a synthetic echo when the mobile caller signal level, also known as the reference signal power, exceeds the ringing level. Consequently, the echo canceller falsely adapts by improperly generating synthetic echo during ringing periods. In addition, when the receiving phone goes off-hook, the echo canceller may initially exhibit echo enhancement or fail to cancel the echo signal.
Problems with prior methods of echo cancellation may also occur because of severe echo path changes. This occurs when the communications network changes from a zero echo path to a finite echo path or vice versa. A typical zero echo path network is a 4-wire phone network. Connections to directory assistance utilize the 4-wire system. Most other connections utilize a 2-wire system that generates an echo signal. The presence of the echo signal in the network creates a finite echo path.
Severe echo path changes are likely to occur if a mobile caller that is connected to a 2-wire landline phone, finite echo path network, is transferred to a 4-wire phone, zero echo path network. Severe echo path changes may also occur if a mobile caller that is connected to a 4-wire phone, zero echo path network, is transferred to a 2-wire phone, finite echo path landline phone. This scenario is likely to occur when a mobile caller is transferred by directory assistance.
The prior art lacks the ability to adequately control the echo canceller to counteract its tendency to falsely adapt during ringing and failure to account for the conditions created when the telephone line goes off-hook. In addition, the prior art does not adequately address changes from zero echo path networks to finite echo path networks and vice versa.